Mirrors for various uses are known in the art. For example, see U.S. Pat. Nos. 5,923,464 and 4,309,075 (all hereby incorporated herein by reference). Mirrors are also known for use in projection televisions and other suitable applications. In the projection television context, see for example U.S. Pat. Nos. 6,275,272, 5,669,681 and 5,896,236 (all hereby incorporated herein by reference).
One type of mirror is a second or back surface mirror (most common), while another type of mirror is a first or front surface mirror (less common). Back surface mirrors typically include a glass substrate with a reflective coating on a back surface thereof (i.e., not on the front surface which is first hit by incoming light). Incoming light passes through the glass substrate before being reflected by the coating in a second surface mirror. Thus, reflected light passes through the glass substrate twice in back or second surface mirrors; once before being reflected and again after being reflected on its way to a viewer. In certain instances, passing through the glass substrate twice can create ambiguity in directional reflection and imperfect reflections may sometimes result. Mirrors such as bathroom mirrors, bedroom mirrors, and architectural mirrors are typically back or second surface mirrors so that the glass substrate can be used to protect the reflective coating provided on the rear surface thereof.
In applications where more accurate reflections are desired, front (or first) surface mirrors are often used. In front/first surface mirrors, a reflective coating provided on the front surface of the glass substrate so that incoming light is reflected by the coating before it passes through the glass substrate (e.g., see FIG. 1). Since the light to be reflected does not have to pass through the glass substrate in first surface mirrors (in contrast to rear surface mirrors), first surface mirrors generally have higher reflectance than do rear surface mirrors, and no double reflected image. Example front surface mirrors (or first surface mirrors) are disclosed in U.S. Pat. Nos. 5,923,464 and 4,780,372 (both incorporated herein by reference).
Many first surface mirror reflective coatings include a dielectric layer(s) provided on the glass substrate over a reflective layer (e.g., Al or Ag). Unfortunately, when the coating becomes scratched or damaged in a front surface mirror, this affects reflectivity in an undesirable manner as light must pass through the scratched or damaged layer(s) twice before reaching the viewer (this is not the case in back/rear surface mirrors where the reflective layer is protected by the glass). Coatings typically used in this regard are not very durable, and are easily scratched or otherwise damaged leading to reflectivity problems. Thus, it can be seen that front/first surface mirrors are very sensitive to scratching. Other possible cosmetic problems associated with first surface mirrors include pinhole formations, corrosion, adhesion, and/or reflectivity level.
For example, prior art FIG. 1 of the instant application illustrates a first surface mirror including glass/Al/SiO2/TiO2, where the aluminum (Al) reflective layer is deposited directly onto the glass substrate. Such mirrors suffer from problems such as poor adhesion, pinholes, poor scratch and abrasion resistance, and other durability and cosmetic problems. These durability problems are particularly evident when float glass (soda lime silica glass) is used as the substrate.
Unfortunately, the durability of first surface mirrors as shown in FIG. 1 is problematic. There is poor adhesion between the metal layer (Al) and the glass substrate. If a metal layer such as Cr is added below the Al between the Al and the glass substrate, corrosion of metal(s) tends to be caused by electrochemical reactions due to the flow of electrons among metals having different free energy if a multiple-layered metal (e.g., Cr/Al) is used to improve metal/glass adhesion. Thus, such first surface mirrors suffer from yield loss on mechanical durability tests due to the delamination of Al from the glass and/or silicon oxide. In a situation where a metal layer such as Cr is added below the Al, between the Al and the glass substrate, delamination of the coating from the glass is improved but the product sometimes fails the salt fog test due to metal corrosion. Thus, it will be appreciated that poor bonding between the reflective metal layer and the substrate can cause failure such as large area delamination, or degraded optical performance such as reflection loss caused by pinholes and/or bulges. This poor bonding is caused by the sharp interface caused by the different categories of adjacent materials such as reflective metal on glass.
It will be apparent from the above that there exists a need in the art for a first/front surface mirror that is less susceptible to scratching, corrosion, pinhole formations, and/or the like.
Instead of a sharp interface between two different materials (e.g., Al and glass), a bonding layer is provided between the reflective layer and the substrate (e.g., glass or plastic substrate) in certain example embodiments of this invention. The bonding or nucleation layer may be of or include elements from both the reflective layer (e.g., Al or the like) and the substrate (e.g., silicon oxide or the like), in order to mitigate the sharp interface between the substrate and reflective layer. In certain instances, the transition can be further smeared by having the composition of the nucleation layer change (e.g., continuously or non-continuously) so as to be graded, so that the nucleation layer is rich in the reflective element close to the reflective layer and rich in the substrate element(s) close to the substrate.
In certain embodiments of this invention, a silicon metal oxide (e.g., silicon aluminum oxide) inclusive nucleation layer is provided under the reflective layer (e.g., Al layer). It has surprisingly been found that providing such a nucleation layer immediately under and contacting the reflecting layer significantly improves durability of the resulting first surface mirror, and the overall optical performance of the mirror. The silicon metal oxide inclusive layer is a good nucleation layer for the reflective material such as aluminum (Al). In certain example embodiments, the metal in the silicon metal oxide nucleation layer is the same metal as in the reflective layer (e.g., Al is in both the reflective Al layer and the silicon aluminum oxide inclusive nucleation layer), thereby improving durability.
Such a silicon metal oxide inclusive nucleation layer may be provided above and/or below the reflective layer in certain embodiments of this invention to improve adhesion above and/or below the reflective layer.
It has unexpectedly been found that the use of such a silicon metal (e.g., Al) oxide nucleation layer above and/or below the reflective layer significantly improves durability of the resulting first surface mirror, with respect to reducing delamination and/or corrosion.
In certain example embodiments of this invention, such first surface mirrors may be used in projection televisions, copiers, scanners, bar code readers, vehicle mirrors, overhead projectors, and/or any other suitable applications.
In certain example embodiments of this invention, there is provided a mirror comprising a substrate supporting a coating, wherein the coating includes at least a reflective layer comprising aluminum, first and second dielectric layers, and at least one layer comprising silicon aluminum oxide, wherein the layer comprising silicon aluminum oxide is located closer to the substrate than are the reflective layer and the dielectric layers, and wherein the reflective layer comprising aluminum is located between the substrate and the dielectric layers; wherein the layer comprising silicon aluminum oxide is located directly under and contacting the reflective layer comprising aluminum in order to improve at least durability of the first surface mirror; and wherein the layer comprising silicon aluminum oxide is graded in at least three different respects so that the layer comprising silicon aluminum oxide (a) contains more oxygen at a location closer to the substrate than at a location further from the substrate; (b) contains more Si at a location closer to the substrate than at a location further from the substrate; and (c) contains more Al at a location closer to the reflective layer than at a location further from the reflective layer.
In other example embodiments of this invention, there is provided a first surface mirror comprising a substrate supporting a coating, wherein the coating includes at least a reflective layer, first and second dielectric layers, and at least one layer comprising silicon aluminum oxide, wherein the layer comprising silicon aluminum oxide is located closer to the substrate than are the reflective layer and the dielectric layers, and wherein the reflective layer is located between the substrate and the dielectric layers; wherein the layer comprising silicon aluminum oxide is located directly under and contacting the reflective layer in order to improve at least durability of the first surface mirror; and wherein the layer comprising silicon aluminum oxide is graded in at least two of the following three different respects: (a) the layer comprising silicon aluminum oxide comprises more oxygen at a location closer to the substrate than at a location further from the substrate; (b) the layer comprising silicon aluminum oxide comprises more Si at a location closer to the substrate than at a location further from the substrate; and (c) the layer comprising silicon aluminum oxide comprises more Al at a location closer to the reflective layer than at a location further from the reflective layer.
In other example embodiments of this invention, there is provided a first surface mirror comprising: a substrate supporting a coating, wherein the coating includes at least a reflective layer, at least one dielectric layer, and at least one layer comprising silicon aluminum oxide, wherein the layer comprising silicon aluminum oxide is located closer to the substrate than are any of the reflective layer and the dielectric layer, and wherein the reflective layer is located between the substrate and the dielectric layer; wherein the layer comprising silicon aluminum oxide is located directly under and contacting the reflective layer in order to improve at least durability of the first surface mirror, and wherein the reflective layer reflects incoming light away from the glass substrate; and wherein the layer comprising silicon aluminum oxide is more oxided at a location closer to the substrate than at a location further from the substrate.